J . Am. Chem. Soc. 1987, 109, 2179-2181
2179
onstrated that the device presented here is very useful in a general sense and rather necessary for discussing the R R spectra of a transient species of enzymic reactions. Registry No. Peroxidase, 9003-99-0.
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A Two-Photon Study of the "Reluctant" Norrish Type I Reaction of Benzil' W. G. McGimpsey and J. C. Scaiano*
Division of Chemistry National Research Council of Canada Ottawa, Ontario, Canada K I A OR6 Received August 25, 1986
t RAMAN SHIFT/crn" Figure 2. RR spectra of compound I (A) and compound I1 (B) of HRP obtained by subtracting the contribution of the ferric enzyme from the transient spectra shown in Figure 1: spectrum A = spectrum e - (0.23 X spectrum d); spectrum B = spectrum f - (0.25 X spectrum d).
note that the relative intensity of Raman lines at 1640 and 1379 cm-l is distinctly different between parts A and B in Figure 2, although their frequencies are unexpectedly close. Previously Van Wart and Zimmers observed the spectrum of compound I in frozen solution with low laser power, but due to its similarity to the spectrum of compound 11, they attributed their observations to a photoreduced species of compound I, that is, compound 11. As noted above, the present sample does not contain compound 11. If compound I was photoreduced to the ferrous state and it reacted with 02,then compound I11 with A, = 417 nm might be formed.I2 However, such a possibility is ruled out from the absence of an absorption band around 417 nm in trace b in Figure 1. These considerations lead us to propose that spectrum A in Figure 2 arises from compound I of H R P and to admit that our cryogenic measurements reported previously yielded the photoreduced ferrous species. HRP compound I has been considered to contain the a2,, porphyrin r-cation radical on the basis of the absorption spectra of M(0EP) derivatives (OEP = octaethylp~phyrin).~~ Recently, Spiro and co-workers14succeeded in categorizing the RR spectra of cation radicals of M"(0EP); the al, radical exhibits positive (14 to 21 cm-l) and negative (-19 to -38 cm-') shifts for the v4 and v2 modes, respectively, whereas the a2, radical exhibits negative (-12 to -22 cm-I) and positive (20 to 23 cm-I) shifts for them upon formation of a cation radical. Accordingly, it was expected that the v4 band around 1380 cm-' of compound I1 exhibited a negative shift in compound I. Nevertheless, there is no prominent frequency shift between parts A and B in Figure 2. One likely reason for this insensitivity is that a clear cation-radical state as seen for M"(0EP) is not generated for compound I. This would mean that the oxidative equivalent is extensively delocalized through iron and axial ligands. An alternative explanation assumes that the shifts expected for the formation of the cation radical of FeIVporphyrin are much smaller than those for M"(0EP). This possibility arised from the facts that the amount of frequency shifts upon the formation of cation radicals are noticeably metal-dependent14 and that there has been no experimental data for iron porphyrins. Thus the R R spectra cannot distinguish between the al, and aZuradicals for compound I. However, it has been dem~
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(12) Tamura, M.; Yamazaki, I. J . Biochem. 1972, 71, 311-319. (13) Dolphin, D.; Forman, A.; Borg, D. C.; Fajer, J.; Felton, R. H. Proc. Natl. Acad. Sci. U S. A. 1971, 68, 614-618. (14) Kim, D.; Miller, L. A,; Rakhit, G.; Spiro, T. G. J . Phys. Chem 1986, 90, 3320-3325.
0002-7863/87/l509-2179$01.50/0
During the last few years the applications of pulsed lasers in industry and in research have increased dramatically. It is now recognized that in some cases laser and conventional (e.g., lamps or sunlight) irradiation can lead to different chemical consequences, even when the total energy and wavelength used may have been the same. A few studies have been concerned with the transient phenomena associated with these and occasionally the "laser vs. lamp" effects on products have been e ~ a m i n e d . ~There ~ , ~ , ~are, however, few examples where products and transient phenomena have been examined as part of the same st~dy.~~,~ The laser vs. lamp differences are frequently related to the involvement, directly or indirectly, of multiphoton processes under laser irradiation. In our work we have found it desirable to identify systems where two-photon processes would take place, but where no monophotonic chemistry occurs. Compounds with these characteristics could find wide application as laser-photosensitive materials, but where no protection from normal illumination (e.g., room lights or sunlight) is necessary. In this paper we report preliminary results on the photochemistry of benzil (Ia), which has been reported to be essentially photostable at room temperaturei0 but which under conditions of laser irradiation undergoes the Norrish type I cleavage, reaction 1. Irradiation of benzil ( 5 X lo4 M in benzene) with the 308-nm pulses from an excimer laser ( - 5 ns, 6 5 0 mJ/pulse)" leads to (1) Issued as NRCC-26853. (2) Bromberg, A,; Schmidt, K. H.; Meisel, D. J . Am. Chem. SOC.1984, 106, 3056; Ibid. 1985, 107, 83. Bromberg, A,; Meisel, D. J . Phys. Chem. 1985,89, 2507. (3) Scaiano, J. C.; Tanner, M.; Weir, D. J . Am. Chem. SOC.1985, 107, 4396. Johnston, L. J.; Scaiano, J . C. J . Am. Chem. SOC.1985, 107, 6368. Weir, D.; Scaiano, J. C. Chem. Phys. Lett. 1986, 128, 156.
(4) Bhattacharyya, K.; Das, P. K.; Fessenden, R. W.; George, M. V.; Gopidas, K. R.; Hug, G. L. J . Phys. Chem. 1985, 89, 4164. Meisel, D.; Das, P. K.; Hug, G. L.; Bhattacharyya, K.; Fessenden, R. W. J . Am. Chem. SOC. 1986, 108, 4706. (5) Johnston, L. J.; Scaiano, J. C. Chem. Phys. Lett. 1985,116, 109. Horn, K. A.; Allison, B. D. Chem. Phys. Left. 1985, 116, 114. Sitzmann, E. V.; Langan, J.; Eisenthal, K. B. Chem. Phys. Left. 1983, 102, 446. (6) Scaiano, J. C.; McGimpsey, W. G.; Casal, H. L. J . Am. Chem. SOC. 1985, 107, 7204. (7) Johnston, L. J.; Scaiano, J. C. J . Am Chem. SOC.1986, 108, 2349. (8) Baumann, H.; Merckel, C.; Timpe, H.-J.; Graness, A,; Kleinschmidt, J.; Gould, I. R.; Turro, N . J. Chem. Phys. Left. 1984, 103, 497. Nagarajan, V.; Fessenden, R. W. Chem. Phys. Lett. 1984,112,207. Topp, M. R. Chem. Phys. Lett. 1976,39,423. Baumann, H.; Schumacher, K. P.; Timpe, H.-J.; Rehack, V . Chem. Phys. Lett. 1982, 89, 315. Johnston, L. J.; Lougnot, D. J.; Scaiano, J. C. Chem. Phys. Lett. 1986, 129, 205. (9) Turro, N. J.; Aikawa, M.; Butcher, Jr., J. A. J . Am. Chem. SOC.1980, 102, 5127. Ferraudi, G. J . Phys. Chem. 1984, 88, 3938. (IO) (a) Ledwith, A,; Russell, P. J.; Sutcliffe, L. H . J. Chem. Soc., Perkin Trans. 2 1972, 1925. (b) Caceres, T.; Encinas, M. V.; Lissi, E. A. J . Photochem. 1984, 27, 109. (1 1) For a description of the laser system, see: Scaiano, J. C. J. Am. Chem. SOC.1980, 102, 7747.
Published 1987 by the American Chemical Society
2180 J . Am. Chem. Soc., Vol. 109, No. 7 , 1987
Communications to the Editor YI
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the generation of its readily detectable triplet state (A, = 480 nm).I2 When the intensity of the transient absorption signals is monitored as a function of the laser dose (adjusted with neutral density filters calibrated and tested for laser use), one observes a nonlinear dependence of the detectable triplet yield on the laser dose (see insert in Figure 1). This curvature is not due to ground-state depletion, since under our conditions 65% of the benzil molecules are excited. The effect is most likely due to the involvement of two-photon processes resulting from laser irradiation of transient intermediates produced during the early part of the laser pulse. Both singlet ( T = 2.6 ns)13 and triplet states are candidates for this effect. The extinction coefficient of the latter is 4 times larger than that of the ground state at 308 nm, and toward the end of the laser pulse, it competes for absorption with the ground state. The effect of triplet-state irradiation on the transient phenomena was studied by using the pulses from a dye laser (-250 ns, 100-500 mJ/pulse), tuned in the 420-520-nm range. Figure 1 shows a comparison of triplet-decay traces under conditions of one- and two-laser irradiation. In the latter, the dye laser pulse (at 5 17 nm where ground-state benzil is transparent) followed the excimer laser after 0.75 MS. The traces were monitored at 470 nm, where the intense light from the dye laser does not interfere, and show extensive triplet-state depletion. These observations suggest that extensive changes take place upon laser photolysis of benzil; however, product studies are somewhat deceiving. UV-vis spectroscopy shows virtually no permanent change, while GC analysis only reveals the formation of traces of biphenyl, some of which is also formed under lamp irradiation at -300 nm. In order to test whether triplet excitation provides an alternate route to the ground state14or if reversible photochemistry involving reaction 1 followed by radical recombination was taking place, we examined the photochemistry of mixtures of Ia and Ib.lS A preliminary examination of the transient phenomena in this system showed it to be essentially the same as for Ia, and UV-vis spectroscopy again did not reveal any detectable chemical change. However, GC analysis showed formation of substantial yields of IC under laser irradiation, but not from a comparable dose from lamps.16 In the one-laser experiment (308 nm only) the yield (12) Porter, G.; Windsor, M. W. Proc. R. SOC.London, A 1958, 245, 238. (1 3) Determined by time-correlated single-photon counting in cyclohexane at 293 K (Acx = 290 nm, A,, > 390 nm); it should be compared with values of 1.95 and 2.1 ns reported in cyclohexane and benzene, respectively: Flamini, L.; Barigeletti, F.; Dellonte, S.; Orlandi, G . J . Photochem. 1983, 21, 237. (1 4) Deactivation via triplet excitation would be a distinct possibility if the upper excited triplet generated by absorption of a second photon undergoes energy transfer to the benzene solvent or intersystem crossing to the upper levels of the singlet manifold followed by relaxation to S, and incomplete intersystem crossing back to TI. (15) A sample containing - 5 X lo4 M each of benzil and 4,4'-dimethylbenzil, stirred by a stream of small nitrogen bubbles, was subjected to a series of 600 pairs of pulses at 308 and 517 nm; the latter followed each 308-nm pulse after a 0.75-ps delay. The 308-nm pulse was defocused and attenuated to 24% of the available power. A control experiment was run in which only the 308-nm laser pulse was used.
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of IC decreased when the laser was attenuated or when the volume of sample irradiated was increased, as expected for a multiphoton process. Under the conditions of Figure 1, the relative yields of IC formed in the two-laser, one-laser, and lamp experiments were Typical con1.0:0.25:undetected (detection limit is 2 X versions were ca. 4%for the two-laser experiment. We attribute the formation of IC to the combination of IIa and IIb produced in the photolysis of the appropriate precursor. In addition, we observed traces of biphenyl and of 4-methylbiphenyl, but no 4,4'-dimethylbiphenyl. Irradiation of Ib alone in benzene gave traces of 4-methylbiphenyl as the main diary1 derivative as well as an even smaller yield of biphenyl. These results (which are not unprecedented)lobsuggest that at least two minor processes involving the solvent take place. Under conditions of lamp irradiation no benzil consumption was detected. In a few experiments in hydrogen-donating solvents, such as cyclohexane, laser irradiation led to permanent change, as expected from the trapping of benzoyl radicals by the solvent. In conclusion, laser excitation of benzil leads to symmetric Norrish type I cleavage, which does not take place upon lamp irradiation. In the two-laser experiment, reaction 1 results from photolysis of benzil triplets, while in the case of the one-laser results, two-photon processes may occur within the singlet manifold, in addition to the triplet photoprocesses characterized by using two lasers. A rather interesting observation is that in the one-laser experiments we were unable to prevent the occurrence of two-photon processes (as measured by the formation of IC from mixtures of Ia and Ib) even when the energy per unit volume was reduced by about a factor of 50 as compared with that normally (16) Lamp irradiations were carried out by using Rayonet RPR-3000 lamps. The irradiation times (typically a few minutes) were adjusted so as to match the irradiation dose used in the laser experiments.
J. Am. Chem. SOC.1987, 109, 2181-2182 used for laser flash photolysis at N R C and elsewhere. As the beam quality and power of modern lasers improve over the next few years, it will become increasingly important to be aware of these effects if typical flash data are to be reliably interpreted. Further, two-photon processes resulting from transient excitation may play an important role in laser ablation techniques if the reactive intermediates generated can compete with the starting material for light absorption. Benzil and its derivatives may find applications as readily available laser-specific free-radical sources. Some of these applications to vinyl polymerization are currently being explored in our laboratory. Finally, we mentioned in earlier studies the difficulties associated with the determination of quantum yields in two-laser experiments. Work is now in progress trying to develop actinometers for two-photon, two-color processes. Preliminary data suggest that efficient chemical processes, such as that illustrated in Figure 1, may actually reflect low (
